Generation of precision microstructures based on reconfigurable photoresponsive hydrogels for high-resolution polymer replication and microoptics

Microstructured molds are essential for fabricating various components ranging from precision optics and microstructured surfaces to microfluidics. However, conventional fabrication technology such as photolithography requires expensive equipment and a large number of processing steps. Here, we report a facile method to fabricate micromolds based on a reusable photoresponsive hydrogel: Uniform micropatterns are engraved into the hydrogel surface using photo masks under UV irradiation within a few minutes. Patterns are replicated using polydimethylsiloxane with minimum feature size of 40 μm and smoothness of Rq ~ 3.4 nm. After replication, the patterns can be fully erased by light thus allowing for reuse as a new mold without notable loss in performance. Utilizing greyscale lithography, patterns with different height levels can be produced within the same exposure step. We demonstrate the versatility of this method by fabricating diffractive optical elements devices and a microlens array and microfluidic device with 100 µm wide channels.

Zhu et al. explore the use of timed UV exposure (Fig. 2) and their DMD-based maskless lithography system (Fig. 3) to produce grayscale lithography of controllable depth.This is an interesting concept that has also previously been investigated in SU-8 based photolithography for generation of topographically complex molds.The authors successfully demonstrate a correlation between exposure dose on the photoresponsive hydrogel and the height of protrusions in the replicated PDMS objects in Fig. 3. Specifically, a quantitative correlation is provided between exposure time and replica protrusion height in Fig. 3c.It is surprising that the correlation apparently is not smooth throughout across the investigated exposure times, specifically having a kink at 200 s.The authors do not discuss this surprising observation of the lack of smoothness.Obviously, such variation could be caused by measurement uncertainties, but the error bars on all measurements are exceedingly small.Importantly, they do not specify what the error bars represent or from how many independent experiments data were acquired to produce the error bars.This applies both to Fig. 2c and Fig. 3c, but especially Fig. 2c where the uncertainty on some data points appears to be on the order of 0.1 micrometer.The authors should include the raw data for these calibration data in a supplementary file for the reader to fully appreciate the reported high precision of the method.
The authors should also inform the reader on how the grayscale calibration data in Fig. 3g was analyzed, and what the experimental uncertainty is on the outcome.First, the height profile in Fig. 3g right-hand sub-figure clearly has a sloping background level with the valley between the two larger peaks not reaching the background level.Second, the exposure levels in Fig. 3g left-hand sub-figure monotonically increase in going left-to-right bottom-to-top, while the resulting replicated features decrease in height from the right-most square in a row to the left-most square in the row above.This is likely caused by inhomogeneous light exposure by the maskless illumination setup, also suggested by the skewed cross-sectional profile of the concentric exposure of increasing intensity shown in Supplementary Fig. 10.The authors should include an analysis of the illumination homogeneity and discuss how that can influence the reported accuracy of the generated recesses in the hydrogel.
The method presented is interesting as an alternative method for micromold manufacturing.However, its severely limited spatial resolution and limited range of depth makes it unlikely to replace existing methods for prototyping of molds, in particular patterned UV light exposure of thick resin materials such as SU-8.Traditionally, shadow masks would be needed to be procured from external suppliers for patterned resin exposure, but direct-write systems are increasingly commercially available.The authors say that it is a disadvantage that conventionally patterned resins cannot be reused.However, the cost of the resin itself is usually not the cost-limiting factor in using photolithography for mold manufacturing, but rather the experimental setup to generate the patterned light.The authors also state that patterning on curved cylindrical surfaces "… is beyond the ability of conventional photolithography technology, which only works on a flat surface."This is correct if using contact lithography, but photosensitive resins on curved surfaces can obviously also be patterned by projection lithography systems similar to the setup used by the authors.
Reviewer #2 (Remarks to the Author): In the present paper, the authors use a photo-responsive hydrogel as micro mold for PDMS molding.When irradiated with a UV light pattern the hydrogel shrinks locally, which allows micro structuring its surface.After being dried, the structured surface can be used for PDMS molding at room temperature.The microstructures can be later erased by being exposed to visible light, allowing the hydrogel to be reused.Simple optics and micro-fluidic devices were fabricated to demonstrate the versatility of the method.This is a very nice and very original contribution to the existing literature, which stands out from the recent publications in this domain, and I am happy to support its publication.
Questions: 1-Line 163: Instead of using the word "modulus", it would help the readers to use "elastic modulus" here.2-The energetic density used in the UV-irradiation and VIS-irradiation steps is indicated in the methods paragraph, but it would make sense to also indicate it in the main text when the different irradiation steps are discussed.3-Line 237: "Figure 4g and Figure 4g" Reviewer #3 (Remarks to the Author): The paper reports on the synergic combination of responsive hydrogels with photomask-assisted UV irradiation or greyscale lithography for the low-cost fabrication of reconfigurable polymeric micromolds to create large area PDMS micropatterns by standard soft-lithography potentially exploitable for different fields of application mainly including microfluidics and microoptics.The real possibility to introduce novel smart fabrication process capable of decreasing time and costs typically required by standard lithography while maintaining the same resolution has a strong soundness in the micro/nanofabrication fields as also demonstrated by the big research efforts focusing on the smart combination of 3D-printing with novel materials.
Undoubtedly, the approach reported is very appealing, it uses low-cost materials and enables for micromolds easily replicable with PDMS.However, before considering the publication, I have some major comments mainly focused on same statements by the authors and with some doubts on the experimental parts.In particular, all the advantages they mentioned for the fabrication process (fastness, low-costs, high resolution), that are the core of the research, are not so evident in my opinion.
1) Fastness: The main point I would like to highlight is that the authors stated that the reported fabrication process is time-saving, especially considering that reconfigurable molds can be used to create many replicas.According to what reported in the Manuscript, summing up the times reported in the materials and methods section, at least six days are necessary to fabricate the polymer molds, engraving it, replicating with PDMS and reconfiguring the molds.Furthermore, these 6 days could be acceptable if the molds could be replicated many times, while the authors stated that they can be used for 4 replicas before degrading.Also from Figure 2 E, from the 2nd to the 4th replicas the shape differences as well as the different surface roughness are evident.
It is also important to take into account that the main proof-of-concepts demonstrating the potential applications of the reconfigurable molds for microfluidics and microlens arrays i) request the use of a physical mask to guarantee the resolution and ii) are obtained by two-steps PDMS replicas, where the 1st PDMS replica from the reconfigurable molds thus requiring an additional step of silanization and further extending fabrication times.In addition, I have some doubts on the possibility to crosslink the PDMS at room temperature in only 6 hours.Standard procedure typically requires at least 24 hours when working at ambient temperature thus further increasing time for replica preparation.
2) Low cost procedure: If I well understood, to demonstrate the potential application of the reconfigurable molds for high-resolution microfluidic and microoptics, the engraving process is based on the UV-irradiation through physical masks.Are these masks fabricated by standard processes?If this is the case, then the presented approach still relies on side expensive masks fabrication processes when higher resolution is needed.
3) High-resolution: the authors stated that they can exploit the procedure for high-resolution polymer replication at lower costs.However, for proof-of-concept applications where the highest resolution is obtained, mask-assisted UV irradiation is needed, and I was wondering how these masks are produced.On the other hand, the resolution decreases when using DMD and digital masks especially with respect to reported literature and considering fabrication resolution that can be obtained by replica molding of polymeric stamps realized by effective low-cost rapid prototyping methods such as 3D printing.Also I am not so convinced on the surface roughness of the replicas as well on the exact shape of the arrays obtained by using the grayscale projection.I would suggest adding SEM images at least to each example of applications as reported for the microlens arrays.I would like the authors to comment on these three aspects that I consider critical.
Regarding the experimental part, I would like to raise some question: 4) What is the difference in wettability between the cis and trans form?I think should be something already known in literature.5) Based on experience of PDMS replicas of plastic molds, I am aware that when replicating from hydrogels or materials not completely cured, there could be a sticky effect that affects PDMS polymerization or that could require longer PDMS curing (again in contrast with the 6h curing process mentioned in 1)).Probably the authors observed this phenomenon as stated in lines 167-168.Furthermore, I would like to ask if the percentage of crosslinker could affect the speed of reconfiguration.

6) Minor comments:
-could you comment on the trend obtained in Figure 3c The authors' approach to PDMS molding is viable, and their characterization method of using white-light interferometric microscopy is appropriate for the compliant hydrogel molds and PDMS replicas produced.
However, the reported spatial resolution is much inferior to other photolithographic techniques commonly used for PDMS molds, such as patterned UV exposure of spin-coated SU-8.The latter can provide nearly vertical side walls for wall heights of hundreds of micrometers.In contrast, Zhu et al. show very gently sloping side walls with depth gradually chaining over dozens of micrometers for a step height of less than two micrometers (Supplementary Fig. 7).This is equally apparent in the authors' approach to producing a microfluidic channel (Fig. 4g-i), where the nominally rectangular channel shape more closely resembles a gaussian channel profile.
Zhu et al. explore the use of timed UV exposure (Fig. 2) and their DMD-based maskless lithography system (Fig. 3) to produce grayscale lithography of controllable depth.This is an interesting concept that has also previously been investigated in SU-8 based photolithography for generation of topographically complex molds.The authors successfully demonstrate a correlation between exposure dose on the photoresponsive hydrogel and the height of protrusions in the replicated PDMS objects in Fig. 3. Specifically, a quantitative correlation is provided between exposure time and replica protrusion height in Fig. 3c.It is surprising that the correlation apparently is not smooth throughout across the investigated exposure times, specifically having a kink at 200 s.The authors do not discuss this surprising observation of the lack of smoothness.Obviously, such variation could be caused by measurement uncertainties, but the error bars on all measurements are exceedingly small.Importantly, they do not specify what the error bars represent or from how many independent experiments data were acquired to produce the error bars.This applies both to Fig. 2c and Fig. 3c, but especially Fig. 2c where the uncertainty on some data points appears to be on the order of 0.1 micrometer.The authors should include the raw data for these calibration data in a supplementary file for the reader to fully appreciate the reported high precision of the method.
The authors should also inform the reader on how the grayscale calibration data in Fig. 3g was analyzed, and what the experimental uncertainty is on the outcome.First, the height profile in Fig. 3g right-hand sub-figure clearly has a sloping background level with the valley between the two larger peaks not reaching the background level.Second, the exposure levels in Fig. 3g lefthand sub-figure monotonically increase in going left-to-right bottom-to-top, while the resulting replicated features decrease in height from the right-most square in a row to the left-most square in the row above.This is likely caused by inhomogeneous light exposure by the maskless illumination setup, also suggested by the skewed cross-sectional profile of the concentric exposure of increasing intensity shown in Supplementary Fig. 10.The authors should include an analysis of the illumination homogeneity and discuss how that can influence the reported accuracy of the generated recesses in the hydrogel.
The method presented is interesting as an alternative method for micromold manufacturing.However, its severely limited spatial resolution and limited range of depth makes it unlikely to replace existing methods for prototyping of molds, in particular patterned UV light exposure of thick resin materials such as SU-8.Traditionally, shadow masks would be needed to be procured from external suppliers for patterned resin exposure, but direct-write systems are increasingly commercially available.The authors say that it is a disadvantage that conventionally patterned resins cannot be reused.However, the cost of the resin itself is usually not the cost-limiting factor in using photolithography for mold manufacturing, but rather the experimental setup to generate the patterned light.The authors also state that patterning on curved cylindrical surfaces "… is beyond the ability of conventional photolithography technology, which only works on a flat surface."This is correct if using contact lithography, but photosensitive resins on curved surfaces can obviously also be patterned by projection lithography systems similar to the setup used by the authors.
In the present paper, the authors use a photo-responsive hydrogel as micro mold for PDMS molding.When irradiated with a UV light pattern the hydrogel shrinks locally, which allows micro structuring its surface.After being dried, the structured surface can be used for PDMS molding at room temperature.The microstructures can be later erased by being exposed to visible light, allowing the hydrogel to be reused.Simple optics and micro-fluidic devices were fabricated to demonstrate the versatility of the method.This is a very nice and very original contribution to the existing literature, which stands out from the recent publications in this domain, and I am happy to support its publication.
Line 163: Instead of using the word "modulus", it would help the readers to use "elastic modulus" here.
The energetic density used in the UV-irradiation and VISirradiation steps is indicated in the methods paragraph, but it would make sense to also indicate it in the main text when the different irradiation steps are discussed.trans Line 237: "Figure 4g and Figure 4g" The paper reports on the synergic combination of responsive hydrogels with photomask-assisted UV irradiation or greyscale lithography for the low-cost fabrication of reconfigurable polymeric micromolds to create large area PDMS micropatterns by standard soft-lithography potentially exploitable for different fields of application mainly including microfluidics and microoptics.The real possibility to introduce novel smart fabrication process capable of decreasing time and costs typically required by standard lithography while maintaining the same resolution has a strong soundness in the micro/nanofabrication fields as also demonstrated by the big research efforts focusing on the smart combination of 3D-printing with novel materials.Undoubtedly, the approach reported is very appealing, it uses low-cost materials and enables for micromolds easily replicable with PDMS.
Fastness: The main point I would like to highlight is that the authors stated that the reported fabrication process is time-saving, especially considering that reconfigurable molds can be used to create many replicas.
According to what reported in the Manuscript, summing up the times reported in the materials and methods section, at least six days are necessary to fabricate the polymer molds, engraving it, replicating with PDMS and reconfiguring the molds.Furthermore, these 6 days could be acceptable if the molds could be replicated many times, while the authors stated that they can be used for 4 replicas before degrading.Also from Figure 2 E, from the 2nd to the 4th replicas the shape differences as well as the different surface roughness are evident.
Question 9: It is also important to take into account that the main proof-ofconcepts demonstrating the potential applications of the reconfigurable molds for microfluidics and microlens arrays i) request the use of a physical mask to guarantee the resolution and ii) are obtained by two-steps PDMS replicas, where the 1st PDMS replica from the reconfigurable molds thus requiring an additional step of silanization and further extending fabrication times.
Question 10: In addition, I have some doubts on the possibility to crosslink the PDMS at room temperature in only 6 hours.Standard procedure typically requires at least 24 hours when working at ambient temperature thus further increasing time for replica preparation.
Low cost procedure: If I well understood, to demonstrate the potential application of the reconfigurable molds for high-resolution microfluidic and microoptics, the engraving process is based on the UV-irradiation through physical masks.Are these masks fabricated by standard processes?If this is the case, then the presented approach still relies on side expensive masks fabrication processes when higher resolution is needed.
High-resolution: the authors stated that they can exploit the procedure for high-resolution polymer replication at lower costs.However, for proof-of-concept applications where the highest resolution is obtained, maskassisted UV irradiation is needed, and I was wondering how these masks are produced.On the other hand, the resolution decreases when using DMD and digital masks especially with respect to reported literature and considering fabrication resolution that can be obtained by replica molding of polymeric stamps realized by effective low-cost rapid prototyping methods such as 3D printing.Also I am not so convinced on the surface roughness of the replicas as well on the exact shape of the arrays obtained by using the grayscale projection.I would suggest adding SEM images at least to each example of applications as reported for the microlens arrays.
I would like the authors to comment on these three aspects that I consider critical.
What is the difference in wettability between the cis and trans form?I think should be something already known in literature.The authors have addressed the experimental and analysis concerns raised in my initial review.I am still not convinced that their approach will find broad application for replica molding of PDMS, but they offer an alternative method to an established field of research and it may find use for optical components as proposed by the authors in the revised manuscript.
As a follow-up from the first review, the authors should still update the captions of Figures 2 and 3 to indicate the meaning of the error bars (SD, SEM, CI, …) in Fig. 2a-c and 3c as well as the number of samples investigated to calculate the error bar for each data point.
As a minor comment, it is still not entirely clear to me how the authors distinguish the need for a clean room to perform light-based patterning of SU-8 or their hydrogel material, since particulate contamination seems to be an equal problem or non-problem in both cases, depending on the required feature sizes and acceptable areal defect rates.It is not a strict requirement, but it would be nice if the authors could comment on why their approach would be more tolerant to ambient particulate contamination than other microfabrication processes for PDMS molding.
Reviewer #3 (Remarks to the Author): Even if not still convinced on the fastness of the overall procedures, I am satisfied with the response and the corrections.As a minor the caption in Figure 11 in supporting information is not correct (should be a-d instead of a-f).Then, I recommend its acceptance at the current stage.
In line with the requirements listed in author checklist, we have included the following changes to the draft.

Question4. The final paragraph of the Introduction must begin with a phrase
like "In this work" or "Here, we show", and contain a brief summary of the major results and conclusions of the current work, written in the present tense.
Answer4: we have included the following changes to the introduction.
In this paper work, we present a facile to generate micromold displays based on the photoresponsive acrylamide/azobenzene-cyclodextrin (AM/AZO-CD) hydrogel prepared via thermal radical polymerization and their usage in polymer replication.
Question5.Please divide the Results section into subsections, each with a title of 60 characters or fewer including spaces.
Answer5: we have included the following changes to the result part.
? -the caption 4a and 4b should be inverted, and at line 223 the reference for figure 4f should be changed with 4c.Zhu et al. present a method to produce microstructured molds for silicone (PDMS) molding in a photoresponsive hydrogel by patterned light illumination.The manufacturing of the photoresponsive hydrogel material and its ability to reversibly de-swell on local UV illumination and re-swell on white light illumination has previously been reported by Kuenstler et al. (ref.35) using a DMD-based maskless lithography exposure system as also employed by Zhu et al.The novelty in the current manuscript is the application of the structured hydrogels to shape PDMS object surfaces, with the resulting PDMS objects directly functioning as devices or being used for further molding of second generation PDMS replicas.et al.
of PDMS replicas of plastic molds, I am aware that when replicating from hydrogels or materials not completely cured, there could be a sticky effect that affects PDMS polymerization or that could require longer PDMS curing (again in contrast with the 6h curing process mentioned in 1)).Probably the authors observed this phenomenon as stated in lines 167-168.Furthermore, I would like to ask if the percentage of crosslinker could affect the speed of reconfiguration.Could you comment on the trend obtained in Figure3c?et al. et al.The caption 4a and 4b should be inverted, and at line 223 the reference for figure 4f should be changed with 4c.
The sample size (n) must be stated in the corresponding figure legend and data points should be shown for plots with n < 10.For larger sample sizes, please consider box-and-whisker or violin plots as alternatives.Measures of centrality, dispersion and/or error bars should be plotted and described in the figure legend.Answer6: we have included the following changes to the Fig.2and Fig.3.

Fig. 2
Fig. 2 Optimization and usage of the micromold display.(a) Increasing crosslinker ratio results in AM/AZO-CD hydrogels with a higher elastic module; (b) Higher crosslinker ratio causes a decrease of the engraving speed on the hydrogel, but facilitate handing of the micromold displays; consequently, the hydrogel formulation with 1.6 mol% crosslinker was chosen for further experiments; (c) Engraving height increases with increasing exposure time, reaching 10.6 μm after 15 min UV irradiation; (d) Illustration of consecutive setting/resetting cycles of the micromold display with different topographies; (e) Optical pictures and WLI characterization of four PDMS substrates replicated from micromolds generated on the micromold display with a

Fig. 3
Fig. 3 Micromold display structures generated using digital light processing (DLP).(a) Scheme of maskless projection lithography system based on a digital micromirror device (DMD); (b) WLI profile of replicated PDMS with a triangle array; (c) Engraving height increases over exposure time reaching 11 μm after 10 min of